Cancer incidence and mortality rates may be used to test hypotheses regarding cancer etiology that are suggested by other scientific studies. There is some evidence that sunlight exposure may cause immunosuppression, which in turn has been associated with increased risk of non-Hodgkin's lymphoma. Maps presenting the geographic patterns of mortality from non-Hodgkin's lymphoma, melanoma, and nonmelanoma skin cancer were compared. Rates of two types of skin cancer known to be related to sunlight exposure were higher in the south than in the north, whereas non-Hodgkin's lymphoma rates were higher in the north than the south. Mortality rates for the skin cancers were higher in those states with elevated ultraviolet-B radiation levels, but non-Hodgkin's lymphoma mortality rates were lower in those states. These findings provide no support for the hypothesis that sunlight exposure increases the risk of non-Hodgkin's lymphoma. In response to reports of an excess of left-sided breast cancer among women, our analysis of more than 250,000 cases from the Surveillance, Epidemiology, and End Results (SEER) program confirmed an overall 5% excess that occurred for all races and stages of disease, and increased with age for invasive disease. No excess was apparent among men. Geographic variation in the prevalence of risk factors for breast cancer accounted for much of the geographic variation in breast cancer mortality rates in the United States. Ovarian cancer risk decreases with increasing parity and increasing duration of oral contraceptive use. Ongoing analyses are investigating the relation between ovarian cancer mortality trends and the prevalences of these risk factors, which have changed over time in the United States.